The present invention relates to ellipsometric systems for measuring feature configurations on wafers. Particularly, the present invention relates to a compact ellipsometer having multiple measurement beams. The measurement beams operate alternately and in conjunction with a rotating stage such that linear stages for positioning the wafer may be operated within reduced travel ranges.
Semiconductors are typically fabricated by depositing and etching a number of layers that are shaped and configured on the upper or top surface of a wafer. Controlling those fabrication steps and testing the wafer early during production helps to keep production costs low. An increasingly important technique for a non-destructive measurement of semiconductors is ellipsometry. In ellipsometry, a specifically configured probe light beam is directed to reflect off the wafer. The change in polarization state of the beam induced by the interaction with the wafer is monitored to provide information about the wafer.
Ellipsometers have been used extensively to monitor thin film parameters such as thickness, index of refraction and extinction coefficient. More recently, ellipsometers have been used to monitor the properties (critical dimensions) of small, repeating, periodic structures on wafers. These periodic structures are similar to a grating and the measured data can be subjected to a scatterometry analysis to derive information about the structure. Information of interest includes, but is not limited to, line width and spacing as well as sidewall profile.
Such periodic structures have distinct orientations. It has been found that the most useful information about such structures can be obtained if the probe beam of the ellipsometer is directed substantially perpendicular to the line structure.
As seen in FIG. 1, a typical wafer W will have multiple such periodic structures PS formed thereon. In some cases, all of the periodic structures will be oriented in the same direction (i.e. all lines parallel). In other cases, some of the structures will have lines running perpendicular to other structures.
A conventional, ellipsometer is typically provided with a stage for moving the wafer through full linear motions FX, FY as well as rotation about the central axis so that the probe beam PB can be directed to each of the periodic structures PS in the appropriate direction (usually perpendicular to the line structure). The linear motions FX, FY are about equal to the wafer diameter WD. The wafer W moved during the measurement consequently occupies a travel envelope LE that extends in the directions of each of the linear axes about twice the wafer diameter. The travel envelope LE determines the minimal footprint of an ellipsometer apparatus.
Recently, there has been a push to substantially reduce the size of ellipsometer apparatus. This effort is particularly directed to allowing an ellipsometer to be incorporated directly into a semiconductor processing tool. To achieve the desired miniaturization, stage system have been developed which reduce the total range of motion of the wafer, thereby reducing the travel envelope and consequently the footprint of the system (e.g. stages that implement a cylindrical coordinate system consisting of a linear and a rotational stage). The use of these stages has not significantly impeded the measurement of thin film parameters since such measurements are not effected by the direction in which the probe beam strikes the sample. However, such reduced motion stage systems have caused a problem with measuring periodic structures where the impinging direction of the probe beam PB has to correspond to a measurement relevant orientation of the periodic structure PS.
As shown in FIGS. 2A and 2B the wafer can be manipulated with the X- and Y-stages such that only one of the four quadrants of the sample is located at the intersection between the sample and the probing beam. To perform measurements within the other three quadrants of the sample, the rotating stage needs to move by multiples of 90xc2x0. To achieve perpendicular orientation of the periodic sample structures in these cases, a second probing beam perpendicular to the first one is necessary.
This difficulty can best be seen in FIGS. 2A and 2B. In FIG. 2A, the probe beam PB is shown striking periodic structure PS1 perpendicular to the line structure. When the operator wishes to measure periodic structure PS2, the rotating stage is used to bring the sector of the wafer where that structure is located within the region which can be reached by the probe beam. As noted above, the stage travel in the X and Y directions is not sufficient to bring the structure PS2 under the probe beam without a rotation. Unfortunately, and as seen in FIG. 2B, the result of this rotation is to orient the periodic structure PS2 so that the probe beam impinges thereon in a direction parallel to the lines. As noted above, it has been found that most relevant information can be obtained when the beam strikes the structure perpendicular to the line structure.
Accordingly, it would be desirable to develop an ellipsometer system, which can utilize a reduced motion stage but also provides for optimal measurement of both thin film parameters and periodic structures.
In the present invention, a probe beam is selectively directed along two or more beam paths to provide two or more focused beams impinging in various impinging directions on a tested wafer. Two perpendicularly operating linear stages provide a travel area that is only a fraction of the wafer size. Combined with the linear stages is a rotating stage positioned with its axis of rotation perpendicular to the movement plane defined by the linear stages. The movement plane is preferably parallel to the top of the fixed wafer. The rotating stage rotates the fixed wafer within a rotation range such that a number of sectors of the wafer top are brought within range of the respective focal spot during consecutive sector measurement steps. During a sector measurement step, only the linear stages are operated to move the wafer along the focal spot.
The focused beams are positioned in a number and in an angular orientation to each other that corresponds to the number of angular orientations of patterns within the measurement sectors. In the preferred embodiment, where wafer patterns have one measurement relevant orientation, two focused beams are provided in perpendicular impinging directions relative to each other such that the pattern can be measured in all four quadrants while still maintaining a perpendicular orientation of the pattern relative to the plane of the probing beam. Since two focused beams are utilized, the rotating stage operates only to adjust the wafers global orientation prior to the sector measurement steps and to rotate the predetermined sectors within the travel area so that is accessible by the focal spots. In the preferred embodiment, four sectors are defined for measuring a wafer in four consecutive sector measurement steps. The linear ranges of the linear stages are about half the wafer diameter, which significantly reduces the travel envelope and consequently the footprint of the apparatus.
The goal of the subject invention could be achieved using two completely separate ellipsometers mounted on the same support system. In other words, two light sources, two sets of focusing and collecting optics, two sets of polarizers and analyzers and two separate detectors could be used. However, in the preferred embodiment, only a single light source and a single detector are used. This approach not only conserves space, but also reduces complexity as only one light source and one detector needs to be adjusted and characterized for the measurement.
Preferably, a broad band light source is used to generate a polychromatic probe beam. At some point before striking the sample, optics are provided for either splitting the beam along two paths or selectively directing the beam along a first or a second beam path. After reflection off the sample, optics are provided before the detector to either recombine the previously split beam portions or selectively combine the two beam paths into a single path. In the preferred embodiment, movable mirrors are used to create two beam paths rather than splitting the beam to maximize the light energy being used for the measurement.
The subject invention is not limited to any particular ellipsometer configuration. Those skilled in the art will be aware of many variants such as rotating polarizer (analyzer) or rotating compensator systems. The elements necessary to create the polarization state of the incoming probe beam and analyze the polarization state of the reflected beam can be located in the common path regions or in the separate path regions. If in the separate path regions, two sets of optical elements are needed as shown in the preferred embodiment illustrated herein.
In the illustrated embodiment, a broadband rotating compensator (waveplate retarder) system is shown. Such a system is disclosed in U.S. Pat. No. 5,973,787. A suitable rotating analyzer system is shown in U.S. Pat. No. 5,608,526. See also, U.S. Pat. No. 6,278,519. All of the above patents are incorporated herein by reference.
In the illustrated embodiment, a pair of movable mirrors are provided for controlling the beam propagation. In a first position, the mirrors are located out of the beam path and allow the beam to travel along a first beam path. In a second position, both mirrors are located within the beam path and cause the beam to travel along the second beam path. The moveable mirrors are specifically configured to provide high position accuracy and repeatability for a large number of switching cycles.
In the preferred embodiment, stepper motors that have a hollow shaft are utilized to rotate and control the waveplates. The stepper motors are placed such that the beam paths run through the hollow shaft of the stepper motors"" axes of revolution. The hollow shaft assembly reduces significantly the space otherwise occupied by the mechanism for driving the waveplates contributing to a reduced footprint of the optical assembly. As a result, the optical assembly fits into the apparatus despite the increased number of individual optical components.
The combination of single light source and single detector in combination with reduced travel range, multiple alternate focused beams, hollow shaft waveplate assembly and moving mirrors provides for an ellipsometry apparatus that has a footprint smaller than the travel envelope LE for a given wafer size.
As noted above, the subject invention allows periodic structures to be measured from a preferred direction while using a reduced motion stage. It should also be noted that this system allows any measurement area to be measured from any direction. While it is generally true that maximum information may be obtained when measuring a periodic structure when the beam is directed perpendicular to the line structure, additional information may be obtained from measurements where the beam is also directed parallel to the line structure or even at a 45 degree angle with respect thereto. In cases where the measured structure is rather simple, this information alone might even be sufficient. The subject invention allows measurements from any desired direction. Such measurements could be used individually or combined in a regression analysis to more fully characterize the structure.